The invention belongs to the field of macromolecule material modification aids, and particularly relates to a reactive type light stabilizer and a preparation method thereof. The preparation method comprises the following steps: firstly utilizing cinnamate as a reactive group to react with hindered amine to synthesize the reactive type light stabilizer. The light stabilizer contains double bonds and can be bonded onto a carbon chain of a polymer by virtue of chemical reaction; a benzene ring in a cinnamic acid molecule in the light stabilizer and the double bonds form a pi bond of delocalization; in addition, the benzene ring and the carboxyl have appropriate steric hindrance, not only can the light stabilizer be prevented from being self-polymerized, but also the light stabilizer can be ensured to be successfully grafted onto a polymer main chain; in the synthetic reaction process, no polymerization inhibitor is needed, so that the purity of the product and the aftertreatment difficulty are improved; and the raw material cinnamate is wide in source, and not only can be extracted from plants, but also can be synthesized in a chemical method. The reactive light stabilizer and the preparation method thereof are safe, environmentally friendly, low in price and widely applicable to the industrial production.